Ahhhhh I got it now.
let's break it down to basics: you need two L3outs because you have to separate the VRFs on the firewall. That means, on ACI, you will need to create the two L3Outs and for communication with the firewall you will be using two distinct vlans, one for each L3Outs.
Now from perspective of the L3Outs, you can use the same L3 domain, pointing to a single vlan pool, which contain both vlans used for l3out neighborship).
if you already have the vlans used for L3out neighborship in separate vlan pools, then sure, you can use different L3 domains without any problems.
if you have problems with the cfg, and the communication over L3out does not work, share the config you have, plus check if you have any faults.
Stay safe,
Sergiu
