you should be alright as long as you dont crank for to long and you make sure that the car battery is well charged. When the bike is running the charging system is regulated and fused so it will not overload. The ideal thing to use is a mains powered car or bike starter then you can just connect that to your bike battery and then after its started it will not draw any amps from the bike only the bike battery will.

If leaving a lead acid battery for a prolonged period like 2 months you should really use a battery conditioner that cycles the battery and keeps it fresh.