I read the OP a few times just to make sure I wasn't missing something. He didn't actually say he connected the two Ground (GND) leads to two different places in the circuit but that must have been what he did.
So he knew Channel 1's flying lead on the Scope Probe is connected to ground (GND) and to the Mains (he didn't say Mains Earth but I'm sure that's what he meant) but the alarming thing is, he didn't know (or realised) that Channel 2's flying lead on the Scope Probe was connected to the same ground (GND)..... Really?
I hope he realises now, that all those grounds (GND) are connected together and to the metal cover/chassis of the Oscilloscope (if it has one) and to the Mains Earth?
I would say if he doesn't, then he should put the Oscilloscope aside and read the Instruction Manual for the Oscilloscope, also a beginners guide to using Oscilloscopes.
Playing with Circuits and Test Equipment when you don't understand the limitations or the safety aspects of either, is dangerous.
Before you do anything always ask yourself "Do I understand what I am doing and if I do this, is it safe, what will happen?"
Until then, stick to your Multimeter and certainly don't try to test anything with High voltages or High currents.
Some people should have a warning label attached, which reads "I maybe dangerous to my own health"
And if you think I'm being a bit OTT, what could happen, if one day he thought powering his Oscilloscope from an Isolating Transformer (to achieve Mains isolation) was a good idea! BTW you should never do this as it is a potential safety hazard, to you and anyone around you.
I realise this information may not pertain to people with years of knowledge but I don't believe the OP falls into that category.