That’s actually user error and not true.
There are three states for each mod. Alt which on En uS keyboards equates to both alt keys. For European keyboards it generally is the same but there are some quirks but sometimes could be left only. There is lalt and ralt which are left and right respectively.
The error here is that order matters
/cast [mod:alt] spell1; [mod:ralt] spell2
Spell 2 will never be cast as the any alt before it takes precedence For this to work you need a
/cast [mod:ralt] spell2; [mod:alt] spell1
Now you can combine alts and shift etc but again order matters
/cast [mod:alt] spell1; [mod:ctrlalt] spell2
Spell2 in this case will never fire as the alt before it has taken precedence so it needs to be
/cast [mod:ctrlalt] spell2; [mod:alt] spell1
Now also consider the vertical stack order in each action block. Top down first, then for each line it’s left to right. (This is why the Compiled Template button in the template editor is so important to see and solve problems)
/cast [mod:alt] spell 1
/cast [mod:ctrlalt] spell2
/cast [mod:ralt] spell3
The alt on line 1 takes precedence over the ones below it so it needs to be
/cast [mod:ctrlalt] spell2
/cast [mod:ralt] spell3
/cast [mod:alt] spell 1