Will two 5mg pills be as effective as one 10 mg pill cialis?

The short answer is yes. The effect should be the same.

You can take 10mg if you are taking Cialis as needed but if you are taking it on a daily basis without regard to sexual activity then the recommendation is 5mg. Here is info on the dosing of Cialis:

CIALIS (tadalafil) for Use as Needed
The recommended starting dose of CIALIS (tadalafil) for use as needed in most patients is 10 mg, taken prior to anticipated sexual activity.
The dose may be increased to 20 mg or decreased to 5 mg, based on individual efficacy and tolerability. The maximum recommended dosing frequency is once per day in most patients.
CIALIS (tadalafil) for use as needed was shown to improve erectile function compared to placebo up to 36 hours following dosing. Therefore, when advising patients on optimal use of CIALIS (tadalafil) , this should be taken into consideration.
CIALIS (tadalafil) for Once Daily Use
The recommended starting dose of CIALIS (tadalafil) for once daily use is 2.5 mg, taken at approximately the same time every day, without regard to timing of sexual activity.
The CIALIS (tadalafil) dose for once daily use may be increased to 5 mg, based on individual efficacy and tolerability.

The dosing is a little different if you have renal insufficiency or liver problems.