I'm looking at dual SIM handset for my next phone. For the purposes of this question, let's say it works on all the bands of Spark and Vodafone, so coverage won't be an issue.

I have a Spark prepaid SIM with no plan, so everything is on casual rates. I also have a Vodafone on-account plan with data. Both have their own separate Whatsapp account (for one thing), tied to the specific number. SIM-1 would be the Vodafone one (the main one) and Spark would be SIM-2.

My query is if I send or receive a photo or video on Whatsapp (for example) for the Spark number, does it use the data allowance on my Vodafone one to complete the download/upload? Or does it look at the Spark number (where the message is destined for) and say, "you have no data allowance on this number, therefore I'm downloading this at the Spark casual data rate".

Assume that wifi is not available but mobile signal for both networks is.

Sorry for the basic question but dual SIM is something I don't know a lot about.

Thanks :)