Interesting question, but I'm afraid that there's not one simple answer to it.
When you run two reverbs in parallel, the reverb time will be specified by the longer one, more or less.
As soon as you run them serially, there will be many variables which have an influence on the overall reverb time, like their order in the signal chain, the relative volumes, the specific frequency-dependent envelopes and so on. The only thing we can take for granted is that the reverb time will be longer than each of them alone, but most likely shorter than their sum.
In short: Trust your ears! :-)
Kind regards,