Use current phone SIM card to activate old iCloud locked phone?

I found my old iPhone 6 and I have a new iPhone 6. I don't know where the Sim card is for my old one. Can I use the same card from my current iPhone 6 and put it in my old iPhone 6 to activate it and then take it out? After I activate it will I be able to take out the Sim card and put it back the my current phone and have two WORKING phones (one with out and with SIM card) without it deactivating or messing up my current phone?

Yes, that should work fine.

I have a couple of old phones i use for testing things and they don't need the SIM card after the activation stage.

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