I'm not sure if what you want to do is a good idea. You're talking about removing a part of the home that probably supports a LOT of the house! Even if you could do what you want, how will you 'temporarily' prevent the house from collapsing? Also, will the home be structurally sound when you're finished. 'Two pieces of wood' probably won't provide support the same way '1 complete, whole log' does. The 'insert' may get 'forced-out' over time. That is, of course, unless you tie the 2 piece together.
From where I'm standing, I suggest you get in touch with your log home Builder, or contact one for advice. If you have to pay someone some $$$ to just come out and tell you what needs to be done, then that's money well spent. Good Money = Good Advice (hopefully). If you're doing the work yourself, in other words, spend some of the money you save on SOUND advice.