The Sildenafil half-life is around 5 hours, and the Tadalafil half-life is around 18 hours.
Your 100mg Sildenafil is the max event dose, whereas the 5mg Tadalafil is the daily low dose (so you're presumably given enough for once a day). Taking the Tadalafil in the mornings, and skipping it on a Viagra day (to take in the evening) sounds like a plan. Some urologists would say just take the Tadalafil every day anyway, but not being a clinician and not knowing your medical history, I certainly can't tell you that.
Sildenafil works best on an empty stomach and particularly having not eaten too many oils/fats.
Tadalafil doesn't matter what/when you've eaten.
The other ways to have the background continuous dosing while getting some boosts to try for sex is to have 2 x 20mg Tadalafil per week (which is commonly prescribed for prostate cancer patients). 20mg Tadalafil is the event dose (equivalent to the 100mg Sildenafil, but works for much longer, often still the next day), but taking it spaced out twice a week also gives you the background continuous dosing because of the long half-life. If you get only the 5mg daily low dose prescribed, taking 3 of them one day and skipping the following 2 days has a similar effect. Some patients manage to get both 5mg daily low dose, and a smaller number of the 20mg event dose tablets prescribed.
Both Sildenafil and Tadalafil are also blood pressure reducing. If you're on other blood pressure reducing medication (which includes Tamsulosin), taking them all together can be an issue for some people.