Hopefully this is the appropriate section to make my inquiry.
The WRT 1900ACS has dual firmware. By default, when you are on boot sector 1, and you load a new firmware through the interface, I believe that firmware is written to boot sector 2. So is this characteristic controlled by the router hardware or by the current firmware? I already have OpenWRT running on boot sector 1 but I need to flash DD-WRT firmware to boot sector 2. Not sure how to proceed from here.
Ummmh, ask the question of the DD-WRT community?
My question is more relevant to the WRT1900ACS and OpenWRT than it is to DD-WRT. There is a place to flash in LuCI so the question is will it take another firmware other than OpenWRT and if it did, would it flash the opposite boot sector like Linksys normally would?
Both Linksys and OpenWRT will flash the alternate partition on this device when updating the firmware and you can alternate between if you want. If you want to know how to install DD-WRT from OpenWRT to the alternate partition, you have ask there.
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Like RuralRoots said, both Linksys OEM firmware and the OpenWrt firmware will flash to the alternate/other/opposite partition and will then switch booting to be from that, in a round-robin fashion. The current firmware stays intact.
The OpenWrt sysupgrade script has an image metadata check, which will likely reject the dd-wrt image (as it does not contain OpenWrt image metadata). However, you can override that check with the "force" parameter, but then it is your responsibility to know that the image to be flashed is really ok.
If the dd-wrt image can be flashed from the OEM firmware, then it is likely ok, but nobody here really knows. This is the OpenWrt forum... 
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The answer is yes, you can flash to DD-WRT on the Linksys 1900ACS using the LuCl interface. Make sure the existing firmware on the target boot sector is a factory Linksys firmware. Download the appropriate DD-WRT factory-to-ddwrt firmware, select that firmware in LuCl firmware update, and choose to force.
I wanted to do this while testing OpenWRT in case I ran into issues. Just from one day of using OpenWRT I have seen security concerns like unknown IPs on WAN Port and firewall prompting about suspicious connection every time the LuCl login screen comes up. May be nothing but I have DD-WRT to use while I investigate.
Appreciate the help despite the nudges and winks to DD-WRT Forum