Someone who I know who goes to the Novus Ordo & the TLM had this to say: they saw an SSPX priest attending a Novus Ordo Mass in choir.
Does the SSPX allow this? [ I know they are firm against celebration of the Novus Ordo ]
I'd be surprised if this were actually true. An SSPX priest generally doesn't have enough time on his hands on Sunday mornings to go and participate in a novus ordo choir - which, from my experience, are nearly universally horrible.
Meanwhile, back in the SSPX, the choir is rehearsing "Mater Dei Misericordae"...
At the same time, the novus ordo choir is practicing "Gather Us In" and "We Are the Bread of Life"....
Not just to bag on the novus ordo choirs, but they do practice sometimes during the week right? I'm not sure an SSPX priest has this much free time on his hands, when you consider the Hours, the rosary, and the prayers and fasting he goes through every day. Meanwhile, charity prevents me from saying what the novus ordo priests are cruising around looking for...