I am a pollinator for Mule palm nursery. You can do it either way. Normally the Butia is the host of the Syagrus pollen because it is a more attractive tree and it is easier as you do not have to get large ladders to climb the syagrus.

The first step would be to remove "emasculate" the male flowers on the butia. If this is done correctly and there are no other butia flowers in the area the only way it will produce seeds is by introducing pollen to the flower. Syagrus pollen is quite easy to introduce to the butia as it falls right off the inflorescence compared to some other palms. The easiest way is to tie the syagrus inflorescence to the butia inflorescence and give it a shake a few times a day. You will see a big dust could of pollen when this is done. If any new emerging flowers come out of the butia why you are doing this make sure you cut them down or emasculate them so there is no cross pollinating back with the butia. Here is a photo of a butia flower that has been emasculate and ready for pollen. There are timing issues as to when it is best to introduce the pollen. Hope that helps.