comp115 (intro to comp science) is skipped in 125 and almost becomes assumed knowledge. For ppl who have no experience with C++, it means they have to do alot of work to cover 125.
Combined comp students do 125 in first yr whereas other actuarial students do 115. The problem you face is choosing whether to do 115 or 125. If you do 115, you have to do 125, so that will prolong the amount of time you will take to complete ur double degree (assuming u transfer after 1st yr into comp..proly a credit/distinction average, 3.5 gpa).
On the other hand if you do 125, you wont waste any time, but you risk doing bad in something you cant handle.
I dunno how much you know about c++. I think if you've done SD in high school then it wont be so bad to do 125 straight away.
If you fail 1 subject, you'll have to do it again next time. No one will force you to drop. However remember that if you fail ur gpa will drop. In later yrs your gpa needs to be abover 2.5 to do some subjects, if you fail alot of subjects your gpa wont be high enough. Lecturers will advise you to drop. But in the end it's your own option. You can take 10yrs to finish your degree, but is that worth it?