Hi libbis,
It's always quite a bit of comparing pass prices and ticket prices to get to the most convenient and economical solution.
These are the pass prices (per person):
As you see, a 15-day Swiss Saver Pass is only slightly more expensive than an 8-day Swiss Saver Pass. Therefor, buying a second pass for the remaining 6 days would be relatively expensive. It depends on ticket prices and the granted discounts per pass whether a second pass would still be worth considering. Ticket prices can be found here:
I looked up the prices of 3 options for those last 6 days to cover. The routes may be slightly different from what you intend to do, as I am not sure of all details, but it should come close:
OPTION 1, STANDARD FARES
Day 9:
Grindelwald - Interlaken West (train): CHF 13.20
Interlaken West - Thun (boat): CHF 38.00
Thun - Grindelwald (train): CHF 26.00
Day 10:
Interlaken Ost - Mürren - Interlaken Ost (train): CHF 34.40
Mürren - Schilthorn - Mürren (gondola): CHF 71.40
Day 11:
Interlaken Ost - Jungfraujoch - Interlaken Ost (train): CHF 181.80
Day 12:
Interlaken Ost - Basel (train): CHF 55.00 (Traveling via Basel is the most direct route. Traveling via Zürich is more expensive. Trains to Paris depart from Basel.)
Day 14:
Basel - Zürich: CHF 31.00
This is CHF 450.80 in total.
OPTION 2, SWISS SAVER PASS
If you would have a Swiss Saver Pass for those days as well, the costs would be:
Day 9:
Grindelwald - Interlaken West (train): free
Interlaken West - Thun (boat): free
Thun - Grindelwald (train): free
Day 10:
Interlaken Ost - Mürren - Interlaken Ost (train): free
Mürren - Schilthorn - Mürren (gondola): CHF 35.70
Day 11:
Interlaken Ost - Jungfraujoch - Interlaken Ost (train): CHF 111.75 (free to Wengen, then 25% discount)
Day 12:
Interlaken Ost - Basel (train): free
Day 14:
Basel - Zürich: free
The tickets would thus cost CHF 147.45. The price difference between an 8-day and 15-day Swiss Pass is CHF 67. The total is CHF 214.45.
OPTION 3, JUNGFRAU RAILWAYS PASS
Day 9:
Grindelwald - Interlaken West (train): CHF 2.20 (free to Interlaken Ost)
Interlaken West - Thun (boat): CHF 38.00
Thun - Grindelwald (train): CHF 14.80 (free from Interlaken Ost)
Day 10:
Interlaken Ost - Mürren - Interlaken Ost (train): free
Mürren - Schilthorn - Mürren (gondola): CHF 71.40 (not included as far as I know)
Day 11:
Interlaken Ost - Jungfraujoch - Interlaken Ost (train): CHF 48.00 (free to Eigergletscher, then 50% discount)
Day 12:
Interlaken Ost - Basel (train): CHF 55.00
Day 14:
Basel - Zürich: CHF 31.00
The tickets would thus cost CHF 260.40. The Jungfrau Railways Pass is CHF 200. The total is CHF 460.40.
As you see, using an extended Swiss Saver Pass would be the cheapest option by far. Another advantage is that you only need to pay from Basel for your tickets to Paris, instead of the full lap from Interlaken Ost.
Does this help you picking your passes?