Who was the pioneer behind joint normal distributions I tried googling, but no luck in finding it.
Who was the one to pioneer these ideas? Also, I'm trying to find when Gauss first started doing some real pioneering work on normal distributions.
 A: As mentioned by Matthew Gunn in comments Lagrange used the multivariate normal in 1776 [1] (Memoir on the usefulness of the method of taking the mean) as a limit of the multinomial, described in Stigler (1986) [2]:

the multivariate normal distribution was far from unknown in 1892. One appearance as a limit to the multinomial distribution, could be found in work published over a century earlier by Lagrange (1776).

The next earliest use I'm aware of is Adrain (1808) [3] where he makes use of multivariate normal errors. 
An account of what he did (and some criticism of part of it) can be seen in 
Gorroochurn (2016) [4] (for example you can see him discussing contours of constant probability being elliptical in the bivariate case in the paper). In the same paper Adrain also discusses least squares estimation.
Subsequently it was discussed by Laplace in 1812.
To answer your other question, Gauss did work on the normal distribution in 1809, deriving it and using it with astronomical observations.
[1] Lagrange, J.L. (1776),
"Mémoire sur l' utilité de la méthode de prendre le milieu 
entre les résultats de plusieurs observations; dans lequel on examine les 
avantages de cette méthode par le calcul des probabilités; & ou l'on resoud 
differens problémes relatifs à cette matière."
Miscellanea Taurinensia 5: 167-232. 
[2]  Stephen Stigler (1986),
The History of Statistics: Measurement of Uncertainty Before 1900
Harvard University Press,  pp 316-317
[3] Robert Adrain (1808),
"Research concerning the probabilities of the errors which happen in making observations, &c."
The Analyst; or Mathematical Museum, Vol. 1, No. 4, pp. 93–109.
http://www.cs.xu.edu/math/Sources/Adrain/1808-Analyst.pdf
[4] Prakash Gorroochurn (2016),
Classic Topics on the History of Modern Mathematical Statistics:
From Laplace to More Recent Times,
Wiley
