2 car seats in fiat 500

[Finlaggan]

DS is 4 & almost 17kg & 104cm.
He is in a britax TWE FF.
DD sits next to him in a britax kidfix XP SICT both seats fit fine in my fiat 500 however I'm aware DS will need to move to a new seat relatively soon.
Any advice on what seat I can get to sit alongside DDs in my fiat?

I can't see that another like DDs will fit.

Thanks

[Motherfuckers]

I don't see why 2 the same wouldn't fit.

[Finlaggan]

There isn't enough width in the car. The SICT has side impact bumpers