You can use a y-splitter to do this, but the best succes in a situation like this would come from keeping the subs equidistant and symetrical.
A true dual Sub AVR which will allow you to control distance and trim for both individually would be better (some AVRs just use the single Sub approach with an internal Y).
A better approach given the limits of your AVR would be to implement a MiniDSP 2x4 and Room EQ Wizard to program it. This gets more expensive, but will allow you to manage delay and volume individually along with managing EQ for the subs. This is an advanced approach and will take time and effort as well as extra cost to implement.