two dhcp servers

for that you need to use failover cluster since you are using 2008 R2 which involves standard shared disk and creating failover cluster with enterprise editions etc

That is complicated since you are running DHCP on DCs

Better you can install and migrate DHCP role on 2012 R2 member server and build DHCP software cluster which will provide you redundancy and failover in case required
https://thesolving.com/server-room/how-to-configure-dhcp-failover-on-windows-server-2012-r2/

Found an article for you. Please refer http://techgenix.com/high-availability-dhcp-server/
Let me know if it helps.

At machine boot time client computers broadcast DHCPDISCOVER request.

Whatever DHCP server hears the request, returns an IP.

So you can have as many DCHP servers + they best be on separate networks, from a client's perspective, to have only one DHCP server respond with an IP.

How do devices know which dhcp server they should get its IP address?

It's not up to them.  They aren't "aware" of "which?".

So if you are running 2008 R2 you could also just delay the the response to the DHCP offer on the secondary server.  They will not be clustered and secondary will only respond when the primary doesn't.  As mentioned you can also use a newer OS.  We run 2016 DHCP servers and they sync the data, no clustered storage.  Seems to be working fine the last year or so.  

As for the question of how do they know, David is correct, the DHCPDISCOVER is what the server is waiting for, it will respond.  The endpoint just broadcasts that, google the DHCP Handshake to further understand the process.

Split scope for DHCP has been around for decades. You configure both DHCP servers to hand out IP addresses, but they use non-overlapping pools so that they don't both hand out the same IP address. The DHCP client decides which address to accept (usually the first to respond). I have personally been doing this since at least Windows 2000, and maybe Windows NT 4.

There is no difference in operation between wired and wireless DHCP clients.

https://blog.thesysadmins.co.uk/configuring-dhcp-split-scope-in-server-2008-r2.html

Windows 20008 R2 goes end of support in about 11 months. You should be looking to replace them with new Windows Server in calendar year 2019. The newer versions of Windows Server support DHCP failover and load balancing. Operationally, you would rarely care which DHCP server gave out the IP address.

https://blogs.technet.microsoft.com/teamdhcp/2012/08/06/dhcp-failover-load-balance-mode/