This would depend on how much difference there was between the two mics. If there was only a level difference of a dB or so it would be negligible and could be compensated for. But if there was a difference in the frequency response, then it's an issue. Most matched pairs are matched for frequency response as well as level. In the case of some Russian mics, there can be a vast difference in the frequency response between even several units. Sometimes a large number of these mics need to be gone through to find two that are "matched". And then you are still not assured that these two "matched" have as good frequency response as some of the remaining ones. That is to say to find a pair that exhibits both the best frequency repsonse and is "matched", could be quite a task. After all, who wants a matched pair of mics that don't perform up to spec?

thanx for your fast reply kurt...

i am thinking about to buy a royer 121. in the end i want to use 2 of them to record stereo. but right now i just can affort one. so should i wait until i can buy two at one time? there are a few offers for a matched pair of royers...

thanx again

Royer is a very reputable company with exceptional quality control. I am sure you wouldn't encounter any problems purchasing one mic now and one later assuming that you use caution when using the mic to insure not damaging the ribbon.
Ribbon mics can be very susceptable to damage from excessive air movement that stretches the ribbon. Bag the mic when not in use and even when you move it from one side of the room to another and don't use it on kick drums or right into the bell of a trumpet or saxaphone and you should be fine. With a little common sense, a ribbon mic should last for years with out having to be re-ribboned.

I was under the impression that the Royer ribbons had a lifetime guarantee. Could be wrong,though. I'm pretty sure they make a stereo version too - maybe that would be worth looking into?