While many would prefer to spend a minimum 3nights/4days to get to really see both parks, if a visit to DLP was part of a Europe/Paris trip then 2 days would allow you a chance to visit the key highlights and must do's IMHO.
2 days will be enough to get a taster and to have a great deal of fun. We've been twice, for 5 days each time and have still wanted more each time. 2 days is certainly better than none!
My recommendation would be to get a couple of good nights sleep beforehand, then hit the park as early as you can, that way you will be able to get on the main attractions before most people arrive.
The first tome we wwent was for two nights. You can see an awful lot in that time. We realised that there was a lot we hadn't seen too, when we started reading this forum. We are going for six nights this time, so maybe we will fit everything in!!! Two nights will be agreat intro to DLP, but beware, you will want to go back!!!
i reckon if you don't have young kids 2 days is enough to do all the things an adult would want to .....if you have young kids their exitement is enough that you could spend much longer plus you end up taking twice as long to get everywhere but i would personally would go for any length of time and still think it was worth it not an opportunity to be missed if nothing else it will give you a taster of what its like and trust me you'll be back just more prepared to do everything you never got done the first time round