One of the things with devouring another titan is you gain memories. The Founder titan power is apparently super fucking whacked. You get ridiculous memories and some level of...clairvoyance? When it was said earlier that the 4th season would be a mindfuck, they're not kidding. Eren is kind of spacing out a lot, but because of that we get to see A LOT of what's really going on and it opens up the world itself.
What would happen if they come together? No idea, but it makes you wonder how it will go down. It may very well be the closing act of the series. Although it was seen that the Founder titan power can be activated through touching a member of that royal bloodline, so who knows.
By reading the manga you get the feeling something can happen, but no idea what.