it's hard. REAL hard.
the twin beds and box springs and mattresses must be of exactly the same height.
it's a fix but not ideal. why? because sometimes the beds shift depending on the size of the folks in the beds and how much they move about. imagine the center board collapsing under your guests. happened to mine. imagine the strap that bound the beds together letting go with a snap ... happened to mine.
my suggestion is to spring for big, new mattresses and box springs as soon as you can ... for as many rooms as you can. (spring for ... get it?)
i ran a b&b that had the sorry-est old beds you can imagine. i tried fixes such as you mention, but they weren't great for night after night use. and the most popular rooms had the biggest beds. hands down. complaints in the morning (or in the middle of the night) about the beds were not what i wanted to hear.
you absolutely have to give your guests the BEST beds and BEST breakfasts you can. aside from being scrupulously clean, this will be what brings folks back again and again.
some thought was given to being able to convert the bed to twins and back again to king but we NEVER did. no one was booking looking for the twins in my place. we had one room that kept its twins that was always the last to book and the rest all had one big bed.
so every time i had a little extra money in my pocket, i bought another new mattress/box spring. also the bed frames - tired old metal frames were okay, but i was able to purchase new solid ones each time and give those away. and then i did my craftiest best to make/refurbish handsome headboards. just don't skimp on those mattresses!
