Adding your favourite person as passenger places a significant burden on most Harleys! The stock suspension is bad enough solo but truly shocking when 2-up (pun intended!). Unless you are both svelte lightweights who don't put much weight on your bike, both front and rear are likely to need improvement.

My wife and I have our own solo bikes, but in our Glide we have Ohlins front and rear. I don't know of any budget solutions that will give outstanding results and progressive-rate springs are not a good solution.