Is it good to add a rim around the bottom to help with stress?Very easy. There isn't much difference between joining adjacent glass sides on a single tank to joining adjacent glass sides on two tanks. Very large single tanks often have a split bottom to accomodate flex stress for example.
Have a search through this forum or on youtube if you want to see examples of joined tanks. I've joined 3 and even 4 glass tanks together before.
I live in australia. I found 2 free cracked 4ft tanks i was gonna use and was gonna make a wooden rim for it to help with stressI have never heard of anything like this being attempted. The join of the 2x2' pieces would be incredibly vulnerable to blowing apart from the water pressure.
DISCLAIMER; I am NOT endorsing this, but this is the ONLY way I see it could possibly work;
You may be able to join the 2 bases more easily & safely with an overlapping piece of glass. You could then use the 4 panels of 2'x4' all round for the sides. Your bracing would need to be very sturdy, and you would likely have to construct in its intended location as it would be almost impossible to move safely.
My suggestion is to keep an eye out for a used acrylic tank. There's occasionally 4' wide tanks available. Where do you live? I know of someone who may have an acrylic 6'x4' footprint available in the nearish future...
the tanks are like between 50-60 i think. Is a top brace hard to make?If your tanks are less than 60cm high, you can simply butt join them (silicon glue side edge to side edge) together. If tanks are 60cm or higher you can add an overlapping glass plate to cover the glue seam - the overlapping plate is only for waterproofing and does not need to be same thickness as tank sides or very wide. Don't press the overlapping piece too close to tank glass or too large if you want to cut it out in future.
Don't need a bottom brace but it wont hurt. Does need a top glass brace to prevent bowing - an overlapping glass strip to make the top brace continous or remove old braces and add a new full length top brace.
Don't use wood for structural support.