Pardon the intrusion, but I just wanted to say that I have experience in the area and have some advice.  First off, I wouldn't recommend trying writing multi-algo from scratch, because there are some serious pitfalls with non-trivial solutions.  That being said, the leading options are Myriad and DigiByte.  DigiByte's difficulty adjustment tends to produce wider difficulty swings, but ultimately is better at keeping the block times on target and giving each algo an equal share.  It also provides superb multipool resilience.  Myriad does a fine job as well, though there is an issue I identified with their "work decay" logic that I would recommend fixing if you go that route.