Get new friends.

I perhaps should have just asked how to create a double install then without an explanation, but I'm not paying the ten dollars for Entrenchment unless I can have it both ways; playing online with people who win about minor balance issues doesn't appeal. 

@Kyro, we don't play regularly. We play for a few days once a year or so in the summer, before they get bored and it's over. I can reinstall each time, which is what I've been doing to date, but I have enough room that a double install would be much more ideal for my circumstances. At 3 days (give or take) per year, a 20 dollar investment (or more based on S+H to Canada) would take at least 6-7 years to drop down to a dollar per day level for them. For me, I've already put hundreds of hours into the game and it's worth the money I spent, to them it never would be.

It's possible to launch vanilla Sins with Entrenchment installed (basically you get 2 exes), but Entrenchment requires Sins 1.15, which couldn't be played with pirated games

My friends wanted to play with me.  I told them 1.01 entranchment or nothing.  I won't even play 1.15.  I like the add ons in the expansion and thats it.  It is freaking cheap for the game and super cheap for the exspansion.  Its worth the money even if you only play once a month.  9.95 for an expansion?  two trips to work I pay that for gas alone.  Cofee is 4 bucks in some places.  Gimmie a break.  You drink that once.  They said they won't play than,  I said "see if I care".   I have no issues finding a game online.

I guess theoratically you can install two copies of the game in two directories but I won't know if it will work.  Try it if you care so much.