(1) AMC and APE are not fungible... so we don't have normal arbitraging forces at play. If they were fungible (ie: I could *convert* APE shares into AMC shares) then the spread would narrow very quickly.
Now for my speculation...
(2) In my IB TWS, I notice that APE has 6x the number of shares available to short compared to AMC, so maybe it's sufficiently easier to short via APE to drive the price of APE down more than AMC? (APE has triggered the uptick rule today, whereas AMC hasn't.) Available to short: AMC = 98k; APE = 604k
(3) These are highly speculative stocks. If AMC was some stable company with a long history of paying regular dividends, for example, and if both stocks (AMC and APE) represented equal rights and claims on the company (including dividends) then I would expect a much tighter spread. As is, we have two stocks to speculate on a highly speculative company.... anything could happen. Who knows?... for all I know, most of the trading neophytes who want to go long AMC don't even know there is an APE version so they are disproportionately bidding on AMC relative to APE (so that AMC is down less today). I really don't know. All of this would be arbbed away by more knowledgeable market participants if they were fungible, but they aren't. (Related to this, +1 to FreeGoldRush, below.)
(4) As for the spread, that's a broker-specific question, but I speculate most (all?) brokers are just treating a spread as two separate (unrelated) stock positions.
EDIT: Short fee is currently 23.4% on AMC; 28.6% on APE.