Random Question on VLANs and Tagged Traffic

I have a question about VLANs if anyone has a moment.

SITUATION:

- I have a network called Network1 on VLAN 20

- I have an AP on VLAN 5 broadcasting Network1

- The port on the switch that the AP is connected to is tagged with VLAN 20 and VLAN 5 and set to dual-mode.

- Then let's say we connect a device, DeviceA, to Network1 on that switch. The device is now on VLAN 20.

THEN

- I have a different switch that has all of its ports on VLAN 70. All ports are also set to dual-mode. We connect a device, DeviceB, via RJ45 into one of the ports on this switch. 

QUESTION:

If DeviceA is on VLAN 20 and DeviceB is on VLAN70, how does DeviceA talk to DeviceB? Or rather, how does the switch handle this? Does the port for DeviceB need to be tagged for VLAN20 as well, or does dual-mode handle this? Or, does DHCP handle this exchange as a middleman, takes the message from DeviceA and sends it to DeviceB as untagged, thereby sending network traffic through the default VLAN which is therefore allowed to pass through to DeviceB because the port is set to dual-mode?