Take a piece of heavy gauge sheet aluminum. Form a C channel over the top of the two gearboxes... fold the ends down so that you can use the upper bolts on the gearboxes to bolt the C channel down to each gearbox.
Now the gearboxes support each other and you also have a rigid support running right across the chassis in the middle of your robot. If you don't need the flat surface for mounting things, you can speed hole it.
It's the way we used to do it in the pre-Toughbox KOP days. (But not in the drill motor days.
Where is the smiley with grey hair?)
Jason
P.S. One more word of advice... if you use the gearbox to directly drive one of your wheels then you'll never lose mobility due to a chain coming off. I know... if you do your chains right, they won't come off. But I'll take direct drive any day.