Ask them. If the 2 tour schedules fit they should be willing to put together a package for you.
One catch would be that the two Paris incudeds would overlap in what they cover, but Paris is a great city to be on your own in for a couple of days.
Also be aware that the southern boat ride starts most of a days bus ride out of Paris.
We did both Fecnh river trips about 2 months apart 2 years ago. That worked out nicel for us and cost less because we got the trips at sale prices.