6 Days in Berner Oberland - Spliting Time Between Two Locations?

It was really quite efficient, just hop on the funicular, change to the second, so I would not worry about that. You can mix it up by also using the train. So, it would really just be a personal preference thing as to whether to change accommodations.
There was also plenty you could do from Murren on foot--once there, we only went up to Schilthorn or out on foot.

I asked a similar question last year, so this link might be helpful, and with a search you will find several other relevant posts:

https://community.ricksteves.com/travel-forum/switzerland/question-about-where-to-spend-six-nights-in-the-berner-oberland

And here is my trip report, where you can read about how it worked out:

https://community.ricksteves.com/travel-forum/trip-reports/trip-report-alsace-wengen-murren-lucerne

Edited to summarize: We spent 3 nights in Wengen, 3 nights in Mürren and were happy with that decision. Let me know if you have any questions. More information about what you want to see and do near Mürren and elsewhere might be helpful.

I'm also thinking the trek between Mürren and Lauterbrunnen might get a bit tedious after a few days.

Since there are two different ways to travel this route, you can easily mix it up some. I have taken the new aerial cableway between Stechelberg and Mürren a few times now - it takes only four minutes and is very efficient with four stations to scan your ticket and enter. This means that even longer lineups can move very quickly. Also, you will experience some amazing scenery along the way and, depending on the weather, one day can look quite a bit different from another.

If you like doing scenic walks you can always walk part of the journey as well (Grütschalp- Mürren, Mürren - Gimmelwald, Stechelberg - Lauterbrunnen).

second location (TBD) to explore the rest of the Berner Oberland away from that valley

Getting out of the Lauterbrunnen Valley, there are many options in the BO. It is much larger than a lot of people seem to realize. Kandersteg is a nice 3-night stay, or you might like a lakeside location like Thun (more of a city) or Meiringen, a small town near the Aare Gorge and the Ballenberg Open Air Museum.

Truthfully, you could spend a month going to the many great bases and places in this canton.