Complex sentence: This is stretching the concept of accessory a bit, as it would be equally valid to call the regulator an accessory of the full face mask or helmet, but the two items are closely connected and generally found in use together.
Split into simpler sentences: This is stretching the concept of accessory a bit, as it would be equally valid to call the regulator an accessory of the full face mask or helmet. But the two items are closely connected and generally found in use together.