I'm going on an international flight via Singapore airlines and I was wondering can you strap a smaller bag onto the front of a pack and will it class as one bag?
Whether or not strapping two bags together will make them count as one piece of luggage will depend on the luggage policy of the airline. It is safe to assume that low cost carriers, or other airlines with strict luggage policy enforcement, will try to charge you for the extra piece of luggage. This will also depend on how evident the strapping procedure is. For example there exist some hiking rucksacks which are made up of a large pack combined with a smaller everyday backpack. These can usually be solidly strapped together in a way that renders it hard to distinguish them from one another. Chances are that these rucksacks don't get flagged as being to pieces of luggage.
One trick I've already suggested here on Travel SE is to wrap your construction in plastic film so that the whole thing becomes one unique piece of luggage. Another similar strategy could be to put your two packs into a large duffel bag to be checked in. This will hide your ruse and solve the luggage count problem. Do check that the combined weight of the packs does not exceed your carrier's limitations or else you'll be billed for another reason.