Stucco around lumber

1. No. It may support two aquariums without collapsing, but it will sag substantially. Adding legs in the middle would resolve that problem, but I'd also consider doubling the beams, front and rear. I'd have confidence in that setup as well (though you'll still probably see some sag).

2. Yes, the front and rear beams each carry half the load. This is a rare example of a very evenly-distributed mass.

Other notes:

It appears as though your upper bolts are very near the top edge of the beams. SPF (spruce/pine/fir) lumber of the last few decades is very soft and prone to splitting. It won't take much to rip those bolts out of the upper edge of the lumber. I'd leave at least 1" between the edge of the lumber and the bolt holes, or add two more in between.

SPF lumber can vary quite a lot in terms of strength and stiffness. By using a single beam front and rear you incur some risk that either the front or rear beam will sag more than the other, resulting in a twist in the aquarium. That's what breaks glass. With something like this, rigidity is key, not just strength.

Here's my 55 setup. notice that it really only has 4 primary points of contact, but they're in key locations and provide a flat plane of support.

A caulk rated for outdoor use should be just fine for this sealing job. If there is any fine particulate in the area left over from drilling through the stucco do brush or blow it away before applying the caulk.