Comparing coefficients in Seemingly Unrelated Regressions

I have a panel data set and have estimated two regression models with the same set of independent variables but different response variable. I have done the estimation separately by random effects method. Please note that I have not used SUR method. SO I run the regressions one for every model.
How can I test the difference between the coefficients of the same variables in two models
if I have
Y1= aX1+bX2
Y2= cX1+dX2
Then how can I test the difference between a and c?
In a journal article I saw a simple t-test for the difference between the means of two groups with equal sample sizes, but I think it is not correct
Please let me know if you have any suggestions. I am using SAS

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