I don't fancy yer chances much, to be honest!
Me and Mrs G (MY Mrs G, not Pete's) were planning a 2 week excursion at the end of June (postponed till at LEAST September now) stopping at Long Meg, Castlerigg, etc, for 2 days, then up to Loch Ness via Loch Lomond (with all interesting features in between), then Orkney. So about 4-5 days travelling, 4-5 days on Orkney, then 4-5 days back (via Aviemore, Edinburgh and Newcastle)...
While looking into this, we DID initially consider Callanish, but it was simply too far. You probably CAN do it, but it'd be one HELL of a squeeze. But what do I know? ;o)
I wouldn't fancy it. Depends where you have to come from, I suppose. Wherever you end up, I'm sure it'll be superb! But I reckon you'll have to choose between Lewis OR Orkney...
Incidentally, I'm pretty sure Moth 'n' Jane have taken 2 weeks for their hols...
G x