An HUGE aquarium for one yearling would still be pretty small.
And likely way too small for two.
Also, at that age, they should be permanently separated.
(From what I understand about Sulcatas)
Your better off with something like an xmas tree tote. They are much cheaper and come with lids you can easily modify for your light and heat source. These won't last long for a sulcata though.
There is no standard size aquarium big enough for yearling sulcatas.
They all grow at different rates, so age doesn't really matter. What size are they? I use 4x8' closed chambers to raise them up in. When they outgrow those, its time to move them outside permanently with a heated night box.