Re: After being at current job for 2 months, previous job calls ....
I have not been in that situation but I also had that same fear. After graduation, I was working at a part time, paid internship while searching for full time work. I interviewed for a promising full time position that ended up cutting all hiring down to part time positions only. So they offered me part time. I could work it in addition to my current internship and split time evenly, but it didn't get me any closer to my goal of full time work. And I knew as soon as that full time offer came through from somewhere else, I would be leaving both of them. Problem was, I didn't know if that full time offer would come tomorrow or 6 months down the road.
I panicked and declined the offer because I was afraid I would just be turning around and leaving directly after. I don't necessarily regret my decision because I'm now in a full time job that I enjoy, but the whole situation made me think. I now believe that it's your job and you need to do what's best for you. From the way it sounds, if you don't take this other position you will be regretting it. It may make for some hard (and awkward) times now leaving two months in but it will work out best in the long run. You don't "owe" you current position anything.