I used to be on Ovranette, which I'm told is chemically identical to Microgynon, and I never had any noticeable problems when/after running two packs together. But if I was slightly late with a pill (i.e. taking one later in the day rather than first thing in the morning as usual) I might have gotten a little bit of spotting but nothing substantial. As long as you're disciplined about taking them at the usual time, you should be fine. And the period afterwards, for me, was no different from normal.
nogginthenog: I'd warn against messing about with the pill too much, such as taking only half a pack to bring on an early period, or going on and off the pill randomly. I had a bit of a scare after thoroughly messing with my pill calendar, when a big lump of fleshy lining came out of me with my period and I freaked out. It might not have been connected, but it was still alarming. I can't remember for sure who it was on
KR that said this to me at the time, I think it might have been Skyy, but it was the best advice re the pill that I had ever received: "the pill is a godsend - treat it as such". I remembered that to this day. Even if you suffer from no ill effects, it just gets messy and complicated if you muck about with it, and you'll never quite know if you're protected.
But running packs together is absolutely fine, and as long as it's done by the book there should be no worries.