It's doable - some flocks can manage with two roosters, particularly if there's enough space for everyone - but there's no guarantees that the Silkie hen would pair with the Silkie rooster. It's just as likely that she'll decide to hang with the Leghorn, depending on which rooster most of the other hens hang out with. Hens will bond with other hens first, and then with roosters second.
The only way to guarantee pure Silkie offspring is to separate the pair for a couple of weeks and that's where you'll strike troubles. Your boys are fine now because they've never been apart and I suspect the Silkie is subservient enough to the Leghorn to not cause trouble. If you remove one rooster, there's a good chance the remaining rooster won't let him back in again. Their memories are ok for a week or two but the fact of the Silkie being a boy will override any previous family feeling and he'll treat him like an intruder.
You can then start running two separate flocks but you'll need a couple more hens for the Silkie boy or the sole girl will get very bored of his attentions.
A third option is to have a single flock of hens, and swap roosters in and out sporadically, and only set the Silkie eggs (unless you want crosses, of course
). The bachelor rooster would have to be housed separately, preferably where he can see and be seen by the hens, but not so close that he and the Leghorn can fight through the wire. This makes introductions much easier.