As you said, it is possible to deploy 2 single radio APs close to each other on 2 different bands (2.5 g and 5 G) to simulate dual radio operation but the challenge is band steering. in this scenario ARM can not handle band steering, results, all clients will prefer to connect to 2.4 GHz AP not the AP on 5 GHz.
therefore I would recommend to have a dual radio PAs so that ARM can do band steering and insist 5Gh capable client to connect to 5Ghz.
Hope got some clarity on this,
Please feelfree for any further help on this.