All being desktop version. Aren't I getting the same amount of oscillators with the two Slims as I would a Voyager?
What would I be missing with the 2 Slims that Voyager RME has?

They didn't allow this enquiry on MOOGFORUMS for some reason

Not quite. The Phatty has two oscs, one filter and no noise - well, not as an audio source - it's a modulation source instead. There are ways around that, either by running an external noise source into the Audio In jack (pre-filter) or using the internal noise as moduation for one of the oscs or the filter frequency. Different results, but depends what you want from it. Generally the Phatty is a good bit of kit, not without its foibles and limitations, but easy to navigate, fun to play, and capable of a nice range of monosynth sounds towards the creamy, grunty side. As an example, I made a track using only a Phatty (except for one sound with external white noise) - link here.

What do I think are the Phatty's strengths? Apart from the family sound, you get an arpeggiator, switchable filter poles, reasonably flexible modulation (including routable velocity) , ease of use despite a little menu-work for the less-commonly-used functions... oh, and a nice overload feature, and audio/cv/gate inputs.

Faults, quirks? Audio white noise seems an odd omission, i/o all at one end rather than round the back, menu not entirely wonderfully arranged (but is a compromise after several OS updates) and though the keybed itself is fine for what it is (and better than you'll get from an old Rogue/Prodigy or similar), I've played nicer.

I can't say anything about the Voyager as I've only dabbled in a shop, but I've been using the Phatty for just over 5 years and have always been very happy with it.

Two Phattys would give you a two-voice, two-osc synth, though you could run one into the other's audio for 4-osc mono. I sometimes run my Rogue into the Phatty's audio input (from a MIDI-CV converter), but have been thinking of getting a MF-107 Freqbox for the purpose.