After converting account to multisig you can’t init transaction from it.
So as I can expect you converted desktop account to multisig and you added iphone wallet as cosigner so you got 1 of 1 multisig.
If you want have second cosigner you need generate third address on desktop wallet and make 2 of 2 setup. So you can init transaction from third wallet on desktop and confirm on Iphone.
As a matter of fact mobile wallet don’t support sending and editing multisig (only signing) so I’m not sure if you are now able to init transaction (you need to import private key from mobile to desktop to fix this now).
What I can suggest you is to not test on mainnet and first learn on testnet how it works. I can send you some test xem. And also please watch this video:
I suggest you to create 2 of 3 multisig. 2 of 2 is not very safe because one of cosigners is able to remove second and take control over multisig account.