If I only have large challos and I don’t want to cut them up, can I make hamotzi while holding the two challos and some slices of challa and then just eat the slices? Does that help for being yotze lechem mishna? Or do I have to eat from the shaleim?
The idea of eating lechem mishna, is to bring out the point that Hashem gave us extra manna in the desert, and to show that we are not losing out by keeping Shabbos. Additionally, if possible we should make the bracha and eat from a roll or challah that is whole, because it is a more prominent way of thanking Hashem when making the bracha. It is thanking Hashem that He gave me a whole roll to eat. It is that roll that we should be eating from, and not the slices. You can eat from the slices also, but if you will only at from the slices it is not considered as if the lechem mishna is the whole rolls, because that is not what you cut after the bracha.
O:CH 274-1. A similar case would be, does one have the preference of a shalem if he holds a whole roll and a piece of bread while saying the bracha, and then only eats slice.