It's possible, but definitely not a reliable method of testing for pregnancy. The opk's look for LH in your system, which will have higher and lower levels throughout your entire cycle. Just having 2 lines show up does not mean that it's positive. For it to be a positive opk, the line has to be just as dark as OR darker than the control line. When this happens, it means you have had an LH surge, and are likely getting ready to ovulate.
I did have a close to positive opk when I got pregnant with my son... But I've seen almost positive lines on OPK's right before my AF starts as well, so... No, it's not a reliable method to test for pregnancy.