I have a lot of trouble connecting riffs to my satisfaction, what are the best options to try when trying to fit two riffs together, tips?

Create a riff that has a structure like this:
A A B C

"A" is any riff, played around the open E string, and "B" is the same riff but played around the 3rd fret on the A string. "C" is a variation of the same riff but played on the 5th fret of the A string. This type of riff flows and loops very well, and makes a nice transition.

you cant really calculate something like that, just keep playing and jamming around favorite riffs until you feel comfortable with your instrument, you'll come up with something sooner or later. don't worry about it too much is my advice.

By connecting your riffs, do you mean transitioning to them? You might just need some messing around and such. Eventually it will get to you.

sliding riffs around chromatically is also pretty fuckin' mint. Try playing a riff @ the 6th fret, then slide it all down to the 5th fret then all the way up to the 9th fret then down to the 8th (ALL ON THE SAME STRING). this would technically be "A B C D" though

That's a good way to play around the harmonic minor scale

Transitions are always a pain. Sometimes you don't really need them as other parts of the band will fill that in for you. For instance, you could easily go from riff A to riff B with a 3 second fill in the middle from the drums and not actually need to write a guitar transition for the part. This is the same with the vocals/bass and using them in this manner can prevent two great riffs becoming muddied non-sense because of a horrible transition.

If you absolutely demand that seamless transition, it really comes down to just playing around. Think about where your fretting hand is at the end of the first riff and where it should be to start the second. See the distance you have to travel on the board, think about the difference between the riffs, and try to build-up something between point A and B. You can always look at the scale patterns, as well, between the last and first note of the two riffs and see if anything obvious comes to mind. Personally, I think this is the absolute hardest part when you write material. You can come up with a million great riffs, but if you can't coherently string them together you're going to have some problems. Just takes time, practice, and patience as always.