Following on twins

I'm headed to Spain in a few weeks and will be doing a lot of multi pitch with 2 followers. What are thoughts on following on a single twin rope(7.9) with followers that are about 160lbs. I am aware that there will be more rope stretch. Looking to do this because I have 2 twins and only one single rated rope although not opposed to buying a second. Also want to travel lighter if possible. Thank you all for the help 

From reading up on the subject, it appears to be pretty standard in alpine environments, but I think guides avoid it. More rope stretch and less abrasion resistance are the downsides. If your followers will be at their limits and hangdogging it’s probably a bad idea. If you don’t expect them to take any/many falls it’s probably fine. Personally I’ve gone with triple rated 9.0s for two followers.

If you do go with anything thicker than Beal Jokers you might want to look into a Kong Gigi belay device to save your elbows

IMO:

The main additional consideration is the cut hazard (and maybe also the "not freaking out the follower when they are looking at dental floss holding them up" hazard).

How to manage that hazard depends on a variety of factors including but not limited to the terrain, your ability to protect the followers from cuts, and your followers' ability to avoid cuts (perhaps by not falling).

Personally, 8.5mm half ropes are the thinnest I have ever put a follower on in fifth-class terrain, though I'll admit to being curious about the new 8.0mm aramid-enhanced ropes on the market.

This seems like an area where it is hard to acquire good judgment because it is hard to experience near-misses.   Also, if you are on-sighting and the route is anything other than a big slab it might be hard to anticipate in advance how hazardous the route will be.

There are some limestone multipitch routes that show some unique erosion patterns that include some pretty sharp, almost knife-like edges. It’s nice to have two ropes when leading these. If I were following such a climb on a single 7.9 I’d certainly not want to fall and scrape the cord across the rock. On the other hand, maybe you could get away with it with (maybe) slightly less worry on Montserrat conglomerate or smoother granite around Madrid, depending on the route. I come from the 11mm days, so super thin ropes kinda scare me. Two Beal Operas (8.5mm) are my go-to in a party of three. 

How long are the pitches you'll be climbing?  If no too long, you can tie into the middle of your single rope and then have two tails for your followers to climb on.