Trident : Cannot join two streams if only one is passed through a persistant aggregator.

When i define a topology such :
Spout1 -> persistantAggregator -> Stream -> join
Spout2 -> Join (the same one)

Join never produces data.

To produce data i need to add a persistantAggregator in other stream.
This persistantAggregator aggregate nothing and just repeat input tuple.

I join a java class testing each configuration

thx