I have a 2 x 4 with a 2 x 5 on top of it but I have 2 males on the bottom and 3 females on the top level so they aren't connected by a ramp and the guinea pigs don't have access to one another. The one thing you need to do is reinforce the bottom of the top level so it doesn't bow. I have grids on the bottom of both levels but added a wooden I-beam that runs the length of the top level to keep it level. A 1 x 2 section sticks out on the top level and I reinforced it with diagonal grids to give it extra support. Here are some photos. I'd reinforce with heavy duty zip ties to keep it sturdy.
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