You can use both, installed with a "T" or installed in 2 different places (ie. side of the head and the coolant elbow or ??? any place the coolant flows and a suitable fitting is available. Is the light to remind you to look at the guage??
'45 Ford PU 66 Valiant wagon, leaning tower of power. 79 Chevy C10 w/250 02 PT Cruiser Convertable