Why don't you set both clips to legato mode so that you can press one key to switch to clip 1, and then press another key to clip 2.
I am assuming that you are not considering to do this because the clips are not of equal length, or one is a full song or something... but consider this.
If you have a song that is say 128 measures... and you have a loop that is 4 measures... this would normally not allow you to seemlessly transition between them with legato mode because then you'll get dumped back to the beginning of your song between the first four measures somewhere... but you can always create envelope automation on clip 2 so that it is also 128 measures long.
Now when you switch between the two they can both have the same length...
To setup a proof of concept here is what I did:
1) Setup Clip 1, which is 4 Measures. Assign it to key press "a". In Launch mode set it to Legato, and set quantization to None.
2) Setup Clip 2, which is 1/4 of a Measure. Assign it to key press "s". In Launch mode set it to Legato, and set quantization to None. Now go to Envelopes. For example Clip Volume. Unlink the Clip Volume Envelope and set it's length to 4 Measures. Having any automation here is not required as just unlinking the envelope and setting a length will allow you to make live "think" that you are working with a 4 measure clip now. Neat huh?
3) I set the tempo to 20, just so I could make sure that it was working as intended... but go ahead and start clip 1... when you get bored, start clip 2. When you trigger clip 1 again it will take off where the "4 Measure" clip 2 was at in time.
Issues... well for this simple example... nothing is in sync... damnit! To work around this... I would make a duplicate version of Clip 1 that is set to quantize global, or bar or whatever, and use that to get the party started. Once it is going, then you can freely use your legato'd and quantize none clips and they will stay in sync.
I know that this isn't *exactly* the one key solution you were looking for. But I hope it is simple and handy enough for you to get done what you needed.
All the best, and hopefully you'll realize that there are still a few helpful people around here