Interesting situation, and there's several ways to look at it, mostly a matter of semantics. In this version of NL KC Lowball the game is played with a SB and BB. Any player to the left of the BB who wants to come in pre-draw must at least double the BB, which in this case is a minimum of 800.
1: 1st question then: is this 800 a "call", a "raise", or something else, such as an "open"? It has similarity to a raise (400 over the BB), but also similarity to a call in that it is the minimum amount you can put out and remain in the game. If I'm facing a bet of, say, 2000, then 2000 (the amount to call) is the minimum I can push out.
2: If we treat it as a call or "similar in nature to a call", then placing two same-denomination chips would be a call if removing one leaves less than the amount to call (or more generally, less than "the minimum amount you can put in and remain in the game"). The problem with labelling 800 as a call is that there is no prior bet of 800 so how can it be strictly speaking (semantically) a call? Although we can refer to it as "calling the minimum opener", for example, again a matter of semantics.
3: If we treat it as a "forced raise", if we remove one of the two 500 chips we still have enough to call (the remaining 500 chip is enough to cover the 400 BB). In that case then pushing out two 500's would seemingly be a raise to 1000 total. One problem with this view is that the "call amount" (400) is not a valid bet, so there really is no "call" option in this case, so we're back to the semantic problem.
4: The 3rd possible label is "the opener". 800 is treated not strictly as a call or a raise, but as an opener, which in fact is exactly what it is (good reference: Roberts Rules, Section 11 "Lowball", see "minimum openers'). If we treat this 800 as the "minimum opener", then putting out two 500's silently would be call-like for purposes of calculating what the player is betting, b/c removing one of the same-denomination chips would leave less than the amount to open. We have other situations where betting action is labelled other than a call or raise... for example, an all-in bet of less than a full raise is not labelled a raise but rather an "all-in wager" with important ramifications for subsequent action.
One possible language amendment to clarify this would be a change to Rule 39 to read: "When facing a bet, unless a raise is first declared, multiple same-denomination chips is a call (or minimum opener) if removing one chip leaves less than the call (or minimum opener) amount." Whether the membership would be in favor of such language (or a separate rule) is uncertain.
Putting it in Rule 39 seems to cloud up a rule which is extremely clean as it now reads so perhaps a separate rule pertaining to lowball would be more appropriate: "Rule X Openers. For purposes of betting and raising, games played with minimum openers (such as NL Lowball), the minimum opener amount will be treated as a call [or a raise if that's the association preference] for purposes of interpreting single and multiple-chip betting action"
I personally favor the semantics of treating the required minimum action as a call-like minimum opener as opposed to a raise as I think it more closely resembles player intent. For example, in this specific case IMO the player's intent will more often than not be to make the minimum opener. if the player wished to raise, they would likely put in something appreciably higher than 1000 here as that is just 200 above the 800 minimum. The potential problem with treating it exactly as call-like is that could be seen as prohibiting a larger opener which is between the minimum and what would be a "full raise" above the minimum open. For example, if the minimum open is identical to a call, then could the opening player put out anything other than 800 or at least 1200?? If it's identical to a call then then next legal opening increment would be a full raise over the call (or 1200 in this case), and that's clearly not the case, a player can open for the minimum or anything over the minimum and in that sense the minimum opener is more raise-like.
Interesting case that may reveal a semantic weakness worthy of clarifying at a future Summit.