Master of Two Magics

Per Cradle and the Crescent, it's possible for a sahir to open the Solomonic Arts on a fresh Hermetic apprentice with a lab total of 10, at which point the new super-apprentice can be taught both sets of arts, and the only downside will be the point when the Quaesitores (and possibly the Suhhar too) knock on his masters' door and politely clear their throats, or else the point when said apprentice inevitably falls to diabolism. Is this WAD?

Not sure what you mean by WAD, but the interesting thing is that I see very few problems with it.
There are a few things you can do with Solomonic Arts that you can't do with Hermetic Arts, but most of the very powerful stuff, comes from being able to ask Jinni to perform for you.
Hermetics can simply use Rego magics to summon Jinni and command them, no Sihr needed.

In the end, having multiple traditions (something that HMRE actually had a virtue for, fairly explicitly so) becomes little more than a point sink for XPs.

WAD: Working As Designed.

It means that the apprentice in question is able to, as you said, pull off several tricks that the Hermetics can't do on their own steam (notably, What Jabir Found Inside The Rock, and Solomonic Physic being a bit better than Hermetic Corpus due to breaking the Limit of Energy), and more importantly, the Magical Defenses are much better than Hermetic form bonii. (21 XP for +2 to all social rolls, for example, is something Jerbitons and Trianomae would kill for, a bit of Astrology is cheaper for a Bonisagus than yet more Magic Theory, and you can delay your Longevity Ritual and the associated Warping for several decades with Alchemy and Physic). As you noted, this hypothetical apprentice has no need to learn Sihr, and Sihr isn't learned on an unlock anyway.

These extra powers seem a bit cheap, though I suppose that it's a reasonable payoff for someone who manages to find a sahir willing to unlock and help teach their apprentice, keep the Quaesitores from knocking on their door and keep the Bonisagi from headhunting their apprentice. (Especially as this doesn't benefit the parens directly, unless they're Tremere.)

Don't see why not. Such an apprentice would be very powerful. The most notable advantage would be if the apprentice could get the Sahir warping response. Also don't forget that Sahir spells effectively have three arts added now. Rego+tech+Solomnic Art. Not sure why the Quaesitores would come knocking. At least they wouldn't have anything. You got yourself a pet hedge wizard. No problem with that!