You can simply use the
iwconfig tools on the both your Pis to join to an ad-hoc network.
use the following:
sudo iwconfig wlan0 mode ad-hoc channel 01 essid my-net
You can use your Link-Local addresses to interact with the pis in general.
A smart way to know your local network Pis can be to use:
ping -I wlan0 126.96.36.199
It is a Multicast Address which will give your pings from the Pis which have the same ad-hoc parameters.
This should most definitely give you the
169.254.x.x address of the Pi within the ad-hoc network.
Networking upon next boot
In best case use the following in your (both Pis)
sudo nano /etc/rc.local iwconfig wlan0 mode ad-hoc essid my-net channel 01 exit 0
brcmfmac in to your
/etc/modules file for availability of chipset on boot.
For further help I have a GitHub repository which can set up Pis (any model 2 or 3) in Ad-Hoc mode with IPv6 Link Local Addresses.