Is con ti a valid way of saying "with you"?
Instead of contigo?
The same reason we say he/him in English but not she/shim. It's just wrong. Irregular and weird, but that's the way languages are ;-)
No, it is not valid. It is not "con" + "ti" but it comes directly from Latin "cum tecum". The same for "conmigo" (with me). In Latin there were another similar forms, Portuguese keeps also "con(n)osco" (with us) and "convosco" (with you [plural]).
thank you for making the Latin and Portuguese connections – I enjoy learning these!