Modified Render onto Cement Render

[Citroënbender]

Has anyone experience with the following?

Just to be clear, not talking about acrylic renders but polymer modified cement renders.

Looking at using a bag mix, as the meterage of this job isn’t sufficient to buy raw materials and mix from scratch, and bags of traditional render are no longer common around here. I can’t find any resource that addresses jointing with traditional render. Should I use a cut joint, or can I feather it over a broken edge of older render? It needs to last. There’s no new substrate, all old work, so nothing to unexpectedly shift there.

I’m just a bit gun-shy, as my experiences in the past with modified coatings have been disappointing, although they were furniture finishes.