I have never been in this area, but on the SBB website I found three Interlaken-Montreux connections with changes in Zweisimmen. All come in on track 7 and depart on track 8. If Zweisimmen is like most stations, tracks 6 & 7 share a platform as do tracks 8 & 9; in other words, you will probably have to change platforms. That's doable, but make sure you are at the end of the car ready to get off when the train comes into the station.
When you get on the platform, look for the signs over the tracks to see what track is across the platform from you. If it's 6, then 8 is on the other side of the train. Locate the stairs, go down them and under the tunnel, and come up on the platform beside track 8. No problem; you might even get there before the train does.