With regards to your existing sub... clearly faithfulness is an issue here and you need to keep that in mind. It is not unusual for a Dom/me to have multiple subs as poly is quite common in the lifestyle but it is not to everyone's taste and both you and your sub need to be in agreement about including other subs. Some couples manage it by only having sex (of any form -oral, anal, vaginal) with one of their subs and having a proper, long term, couply relationship with them in vanilla life as well as the lifestyle but only playing with the other subs (you can whip them, tie them, gag them, even make them masturbate themselves or have them watch you have sex but you don't touch them sexually and vice versa). That sometimes makes the distinctions clear - your current sub is your long term partner, this new guy is just a play date.

But since it seems to be a limit for you at present, I'd avoid the whole issue and thereby neatly avoid the ethics of company ink.