Was it the original intention to combine them? Or was it intended the way it was launched (two computers, or three in one case and that's it, simly said).
Basically yes. It was a sounding approach to get more revenue from an, at that time already ageing, 8 bit concept without investing much, while running a small risk of failure, as there was no risk of alienating existing customers
- New Customers could see it as a better 64 they always wanted to buy.
- Existing C64 customers could see an upgrade path without loosing all investment (read games) thy had spend.
Maybe most important
- Both got offered a more semi professional, output related usage.
Especially the later was important to bind customers growing out from playing with a (classic) home computer into every day productivity related computer use.
Despite all the work dedicated users have done, the C64 wasn't a real replacement on for professional PET series. Mostly due the lack of a good keyboard and an 80 character display. The 128 did offer both, and with CP/M as OS it could tap a great amount of existing productivity software - from word processing to databases and much more. While CP/M had as well past it's peak, it was still considered a good choice. Neither the PC nor DOS was as all mighty as it became a few years later.
I wonder if they could get more out of all those peripherals.
Not really as the 128 was for most part just a combination of existing enhancements for the C64 - adjusted for better integration, not anything really new.