Hedging
By @FPL101_Chris,How do you handle the situation when you have one of your attacking players facing one of your defensive players? It's obviously not ideal with one player's goal potentially wiping out the other's clean sheet but there are options.
Let's assume you are not forced to play both and have the squad depth to choose who to start. You need to think about how you think the game will go and what the likely outcome will be. What's the potential for goals or clean sheets? What other options do you have in your squad - do you have safer picks available?
Do you back the attacker, back the defender, start both or bench both? Let's look at each option.
Back the attacker
If you think it's a game where the attacking player has potential to score well then it might make sense to play the attacker and bench the defender who is unlikely to get any clean sheet points. How does he compare to your other defensive options for the gameweek?
Premium, more expensive attackers tend to still score well even against the best defenses so it's probably not worth benching your expensive attacker because of a tough fixture.
Back the defender
If you think there's a good chance of a clean sheet then you may want to start the defender or goalkeeper and bench the attacker.
If you're in doubt this is possibly the riskiest option as attacking players generally have higher points potential.
Backing a defender relies on the team's performance so it's important to look at the team data and how you think the team will do rather than the individual.
Start both players
If you're really not sure how the game will go, e.g. an unpredictable derby, then you can play both as a way of hedging your bets, hoping you will either get attacking returns or a clean sheet. If your attacker's team does score, is it likely to be this player involved or could a goal come from elsewhere? Another player scoring would be the worst case scenario.
Is the clean sheet the defender's only route to points? If they have a chance of getting some attacking returns and it's a high scoring game then they may still be worth playing.
Bench both players
This is only really an option if you have enough squad depth to be able to field 2 less risky players, players who have fixtures with more predictable outcomes. This is the least likely of the 4 options but it has its place.
Ceilings and floors
Backing either the attacker or defender is the high ceiling tactic. It's more risky but if you call the outcome correctly you have the highest points potential.
Playing both players and hedging your bets is the high floor tactic. It's less risky as you have 2 chances of getting it right but as a goal nullifies a clean sheet there's less potential for points.