>>28049212If you're referring to Xerneas Break, then yeah, it's fine to use both. I happen to have that exact deck, so here's my list. I'm thinking about throwing in a 4th Xerneas to add a tad of consistency, but we'll see. I took it out for the Shaymin, but Shaymin adds enough speed to the deck that I'm not convinced the 4th Xerneas is needed.
As far as Spirit Links go, the rule of thumb is one for every Mega. For instance, I run two for the two Mega Gards.
In game, I try to focus on Xerneas since less work goes into getting him rolling, but Gardevoir hits a lot harder for a lot less. The way I see it, Gardevoir is more of a closer whereas Xerneas is more focused on getting the match rolling.
