It can be very stupid question, but i'm very curious about it.
Bitcoin can sign simultaneously with no extra transaction fee. For example, assume that fee is 0.0005. Signing transactions from two address need only 0.0005 , not 0.001.
Ardor can also conduct Multi-Signing?
In Ardor what we call phasing transactions can do much more than multi signature: Phasing
Where can I see 'phasing transactions' details?
In the link I posted on my previous reply.