Use a group rule. Open Rules, click Add New Rule, and choose the group type. Manual lets you pick the players yourself. Automatic applies the same logic across many players at once.
Always include one of a set. Add the players to the primary group and set "use at least one." Every lineup gets at least one of them. Set "use exactly one" if you want one but never two together.
Keep players apart. Add them to the primary group and set "use no more than one."
Pair one player with another. Make it conditional. If the first player is used, then use exactly one secondary player, and put the second player there. Whenever the first appears, the second comes with him.
Automatic rules do the same thing across the whole slate. To pair every defense with a running back on its own team: if exactly one primary player at D is used, grouped by player, then use exactly one secondary player on the same team at RB. Group By decides how many groups the rule makes: one per player, one per team, one per game, or one group total with Group By off. Add a stat requirement to narrow who qualifies, such as RB projected over 8 points.
Save any automatic rule as a manual rule first and open it to see the players it matched. Once it's right, save it as an auto rule so it recalculates every build.
Related: How do I make sure my lineup rules are doing what I want them to? · How do I keep opposing players out of my lineups?