Connect two PVC vents to vent stack

[Loueber]

Hi all. Fast question.
Massachusetts.
I need to connect 2 two inch PVC vents to this 4 inch cast iron vent stack.
I don't want to drill a hole in the roof for a separate 2 inch stack, the roof is too steep to get in top.
Tieing I to this is the only possibility.

Can I use a fernco 4x4x2 y?
Or does it have to be a PVC 4x4x2 with fernco couplings on top and bottom of the PVC y?
There is only going to be 3 feet above the connection through the roof. If I end up cuting it and it sounds freely and I can pull it down, I will replace the top 4feet with PVC .
But in case there is a bracket holding it in at the roof section. I might just splice in the fitting .

 

[Jeff H Young]

Many ways to do it but the cleanest and relatively easy way involves a cast iron 4x4x2 wye and 2 4 inch no hub couplings plus a 2 inch mission "CP200 " the rest would be pvc glue in fittings.