Well, I do not have much experience with Shaykh Abu Hajira in asking about various Madaaris, but the Shaykh did provide valuable informations. Although, I've also found his advises very helpful as well, I meant that I had much more experience with Shaykh Isma'il Moosa; that's all. I just had more interactions with Shaykh Isma'il Moosa, that's the bottom of it.
Hafiz Gee, I'd recommend you still keep the company of Shaykh Taha Karaan, whether you are studying or not under him.
And I think they do not recommend studying two schools together is because realistically you may need to first master your own school to a certain degree before going on to comparative fiqh.