Here's the rub (at least in the US, Canada, Singapore, and the UK) ... if you're working "full-time" for $COMPANY (ie 40h per week (give or take), then you've contractually agreed that from 0900-1700, you're "theirs"
If you work $ELSEWHERE during the same hours, you're double-dipping, and you will be found out - either by replying to an email from the wrong address / with the wrong signature, being in a meeting for #COMPANY when you're supposed to be in a call with $ELSEWHERE
Now, if you can start one job early-ish (say at 0700-1500), and don't mind working the other one late-ish (say 1530-2330) - and they're not "competing" companies, go for it
You're not going to come out ahead on taxes in any country I'm aware of (regardless of what $COMPANY & $ELSEWHERE reports as your salary, the tax officials will be able to add them together at filing time) - $50k + $50k still plops you at $100k of pay (and, therefore, into whatever tax brackets that entails in your locale)
You may (or may not - you have to figure this out for yourself) be better off leaving Europe if jobs like the one you're looking for are too difficult to find
If you work $ELSEWHERE during the same hours, you're double-dipping, and you will be found out - either by replying to an email from the wrong address / with the wrong signature, being in a meeting for #COMPANY when you're supposed to be in a call with $ELSEWHERE
Now, if you can start one job early-ish (say at 0700-1500), and don't mind working the other one late-ish (say 1530-2330) - and they're not "competing" companies, go for it
You're not going to come out ahead on taxes in any country I'm aware of (regardless of what $COMPANY & $ELSEWHERE reports as your salary, the tax officials will be able to add them together at filing time) - $50k + $50k still plops you at $100k of pay (and, therefore, into whatever tax brackets that entails in your locale)
You may (or may not - you have to figure this out for yourself) be better off leaving Europe if jobs like the one you're looking for are too difficult to find