Can I alternate between SENSE C2/C2+ and C5/C5+ cassettes once I have started to use the cassette?

No. You should never start using one cassette type, remove it and insert another cassette type.  

Once the WaterGuru SENSE cassette is inserted and exposed to water, the countdown starts ticking on how long the testing pads on the cassette will last.  

Once exposed to water, the cassette testing pads have a lifetime of no more than 1 month (C5/C5+ Cassettes) or 2 months (C2/C2+ Cassettes).

Also, such an operation is not supported in the WaterGuru app and not supported by WaterGuru Customer Support.  You would never be able to determine how many water measurement tests are left on either of the 2 cassettes.