Technically even if you file an extension and if you OWE money, IRS will still start accruing interest until you pay the money you owe since the last filing date of 07/15/20 this year.
If you have a refund then there will no be a penalty if you could not file the extension, however it is still a good idea to file the extension or the return subsequently as it is your money.
I'm not sure why you need to file the extension single now if you know it will be jointly as it will belong to two people. For some reason if you need to file single then have both of them file the extension just to be sure.
