Making your presence clear without being in everybody’s face all the time, that’s what playing backup is all about.
Using two notes (of a chord) is a good way to do this.

The next examples are cliche accompaniments in the swing blues style.
You can combine them with chords, basslines and fills. Experiment with different rhythms, too!

To play these riffs, you first need to know where the tonic is.
Find the tonic and you’ll know where to start the riff. Sometimes the riff starts on the tonic, no problemo. In other cases, you need to “calculate” the starting point of the riff: one fret back, one string down or two frets up, same string. Find out what works for you.

The two notes you’re playing here sound nasty when played without a bass note or full chord.
They are the two most important notes of a dominant 7th chord: the third and the flat seventh.

When you move this “tritone” interval down 1 fret from the tonic position you’re playing a tritone on the IV chord: the 3rd and 7th. Moving them up one fret gets you to the V chord.

Combined with a bassline it could sound like this.

In this example every bassline pattern comes in blocks of two bars. Each time we play the tritone interval, we approach it from a fret below to create tension (and release).
Bars 9 and 10 are tricky; play the first note of each bar with your ring finger.

The last two bars are a standard bass run to the V chord and back. Play beat three of bar 11 with your middle finger and slide into the D#7-E7 progression.

Note: Inner Logic

The 7th of the I chord (in this case the G in an A7 chord) leads to the 3rd of the IV chord (the F# in D7). The 3rd of the I chord (a C# in A7) leads into the 7th of the IV chord (a C in D7).
This inner logic of a blues chord progression can also be used in your solos. The same logic can be applied to the V-I chord progression.