Fitting of a sum of vectors $Y \alpha = X \beta + \epsilon$ Related question / Motivation
This question is related to the question here. But the difference in this question is that the weights can be different on both sides of the equation.

This question
This question is an extension of linear regression where we find coefficients $\beta$ relating to the model
$$y = X \beta  + \epsilon = \hat{y} + \epsilon$$
The solution that minimizes $\epsilon$ is also maximizing the correlation between $y$ and $\hat{y}$.
What if $y$ is now containing some flexibility as well and we have a model like
$$ Y \alpha = X \beta  + \epsilon$$
To minimize the $\epsilon$ the trivial answer is $\alpha$ and $\beta$ being equal to $0$ but say that we exclude this option? Ie. we could add the restriction that $\sum \alpha_i  = 1$
Find coefficients $\alpha$ and $\beta$ such that we maximize the correlation between $Y \alpha $ and $X\beta $.
We can assume that the columns in $X$ and $Y$ have zero mean.
 A: If the correlation between $Y \alpha $ and $X \beta$ is maximized then the $\beta$ are such that it is the OLS solution of $\alpha Y$ (where we either translate the columns in $X$ such that the column means are zero, or we add an intercept to the OLS regression).
So we have $$X \beta = X(X^T X)^{-1}X^TY \alpha = Y_{\parallel} \alpha$$
the difference is
$$X \beta  - Y \alpha = (X(X^T X)^{-1}X^T-I)Y \alpha = Y_{\perp} \alpha$$
The image below illustrates this in 3 dimensions.

The possible solutions $Y\alpha$ and $X\beta$ are in a surface spanned by the vectors $Y_1, Y_2, ... Y_n$ and $X_1, X_2, ... X_n$. For a given $\alpha$ the solution of $\beta$ that minimizes the correlation is the OLS solution, ie. the perpendicular projection of $Y\alpha$ onto the surface spanned by $X$.
In this example with 3D coordinates we see that the planes intersect and $\alpha$ can be changed such that the vector $Y\alpha$ is entirely in the plane spanned by $X$. And the correlation can be made equal to 1. But in more dimensions, the hyper-surfaces might not need to intersect (except in the point zero where the planes always intersect).

Solution with optimization using a Lagrangian function
Maximizing the correlation between $X \beta $ and $Y \alpha$ is like minimizing the ratio of the length of the vectors $Y_{\perp} \alpha$ and $Y_{\parallel} \alpha$ or $Y \alpha$. The correlation is larger for a smaller remainder $Y_{\perp} \alpha$ while keeping the projection $Y_{\parallel}\alpha$ or total vector $Y \alpha$ the same size.
We can solve this by minimizing the Lagrangian
$$L(\alpha, \lambda) = \alpha^T \, M_{\perp} \, \alpha + \lambda(\alpha^T \, M \, \alpha - 1)$$
where $M_{\perp} = Y_{\perp}^TY_{\perp}$ and $M = Y^TY$
whose gradient must be zero leading to the equations
$$ \begin{array}{}
(M_{\perp}+\lambda M) \alpha &=& 0 \\
 \alpha^T\,M \, \alpha  &=& 1
\end{array}$$
To solve the first equation,  we are looking for a constant $\lambda$ and vector $\alpha$ such that
$$M_{\perp}\alpha = -\lambda M \alpha $$
or
$$(M^{-1}M_{\perp}) \alpha = -\lambda \alpha $$
Which makes the $-\lambda$ and $\alpha$ the eigenvalues and eigenvectors of $(M^{-1}M_{\perp})$.
The solution with the maximum correlation will be related with the smallest eigenvalue $\lambda_v = -\lambda$. This is because we can compute the correlation as
$$\rho = \sqrt{1-\frac{\alpha^T \, M_{\perp} \, \alpha }{\alpha^T \, M \, \alpha }} = \sqrt{1-\frac{\alpha^T \, \lambda_v M \, \alpha }{\alpha^T \, M \, \alpha }} = \sqrt{1-\lambda_v} $$
So an algorithm to find $\alpha$ and $\beta$ is to compute the eigenvectors of that matrix and select the one with the smallest absolute value of the eigenvalue.
Code example
library(MASS)
set.seed(1)
### create X and Y
X = t(mvrnorm(3,mu = rep(0,10), Sigma = diag(10) ))
Y = t(mvrnorm(3,mu = rep(0,10), Sigma = diag(10) ))

### subtract the means (without loss of generality)
X = X - rep(1,10) %*% t(colMeans(X))
Y = Y - rep(1,10) %*% t(colMeans(Y))

### compute matrices
P = X %*% solve(t(X) %*% X) %*% t(X)  ### projection matrix
Ypar = P %*% Y                        ### Y projected on X 
Yperp = Y-Ypar                        ### residual of projection
M = t(Y) %*% Y               ### quadratic form to compute square length
Mperp = t(Yperp) %*% Yperp


### solve with eigenvectors
eV = eigen(solve(t(M)) %*% Mperp)$vectors


### compute Xbeta and Yalpha 
### when putting alpha equal to the last eigenvector
### the last eigenvalue is the one with the lowest eigenvalue
alpha = alpha = eV[,length(eV[1,])]
yfit = Y %*% alpha
xfit = P %*% yfit
corrV = cor(yfit,xfit)


### optimization of correlation
f<-function(p){
  a1 = p[1:3]
  a2 = p[4:6]
  rho = cor((Y %*% a1),(X  %*% a2))
  d = abs(sum(a1^2)-1)^2
  return(-rho+d)
}

a = rep(1,6)
### optimize correlation
mod <- optim(a,f, method = "BFGS",control = list(maxit=1*10^5,trace=1,reltol=10^-16))
mod

### check results of models 
yh = Y %*% mod$par[1:3]
xh = X %*% mod$par[4:6]

### correlation from optimization
### 0.69886626
cor(yh,xh)

### correlation from eigenvectors 
### 0.6988626 (it's the same)
max(corrV)

Intuition
The correlation in terms of the vector $\alpha$ is the ratio of the length of the vectors
$$\rho = \frac{\Vert Y_{\parallel} \alpha \Vert}{\Vert Y \alpha \Vert} = \sqrt{\frac{\alpha^T \, M_{\parallel} \, \alpha}{\alpha^T \, M \, \alpha}} =\sqrt{\frac{\alpha^T \, M^{-1} M_{\parallel}  \alpha}{\alpha^T\alpha}}$$
If we restrict the $\alpha$ to be of a specific size then the alpha that maximizes this product $\alpha^T \, M^{-1} M_{\parallel}  \, \alpha$ is directed along the eigenvector with the largest eigenvalue.
