Tethering SIM-disabled on iPad but SIM can be used to tether on Windows phone? I've got a SIM from a windows phone for which tethering (internet sharing over wifi) works fine. 

When I put the same SIM into my iPad (3rd Gen, iOS 8.1) - it tells me that I need to contact my carrier to enable Tethering. (I have a second SIM for which tethering works just fine on the iPad). 

My question is: Is it possible for tethering to be SIM-disabled on one device - but work fine on a device from other vendor?
Yes, this is entirely possible. Your SIM card number is provisioned by the carrier to include features X, Y and Z based on your rate plan with them. This can include tethering by device type. If you have multiple SIM cards, it is possible they have enabled the feature for one SIM card but disabled it for the other, either by accident or on purpose.

You would need to contact your carrier directly to find out if they have disabled it.