You've found 1-1/2" ply - boy, I wouldn't want to try to hump that stuff around? I've seen some just over an inch thick and it is a bear to carry and place. Ply is fairly heavy, and 1-1/2" stuff would be REALLY heavy. It is true that thicker plywood is stronger than two sheets of thinner stuff that equals the single thicker piece, but you aren't really interested in max strength I don't think if all you are trying to do is get the floors even.
So, I'd opt for the two thinner sheets. For maximum strength, install them both across the joists, but on the top layer, offset it by 1/4 the distance between joists (this means the top layer's end joint will NOT be on a joist - on 16" OC, the end would be 4" from the center of the joist). This provides the maximum bridging strength across the joist top and ensures you don't have two edges aligned that can squeak from deflection. You'll need lots of either ring-shanked nails or screws to mate the two layers together - don't skimp on those. Do use construction adhesive to bond the bottom sheet to the joists, but do NOT use construction adhesive between the two sheets (you want full contact, and it's almost impossible to get that with construction adhesive, and a full-coat of a wood glue just isn't normally worth the effort and the time to spread and place the sheet before it starts to skin over takes practice and speed). Leave the recommended end and edge gaps so there's room for expansion without cupping or tenting, and you should be fine. Assuming the joists are strong enough, this is the same prep as required for a stone tile installation for the subflooring. For maximum strength, use a T&G ply for the layer directly on the joists - regular, square-edged stuff is fine of the top layer.