The heating element will try to dissipate a set amount of power, if you put in under an insulating layer it will operate hotter in an attempt to dissipate the same load.
Some numbers
60w/m2 system with 25°C floor temperature
3cm bed and tiles above mats - Under tile temperature = c. 28°C
2cm rubber above heat mats - under rubber temperature = c. 55°C
150w/m2 system with 25°C floor temperature
3cm bed and tiles above mats - Under tile temperature = c. 32°C
2cm rubber above heat mats - under rubber temperature = c. 85°C
These are the average temperatures under the tile/mat. The temperature of the heating wire will be considerably more as it occupies only a small percentage of the total area.
Overall I think the rubber mat will cause issues with localised overheating, and I'd avoid the two in combination.
Fitz.