Ad-Hoc Connection

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

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) /etc/rc.local file:

sudo nano /etc/rc.local

iwconfig wlan0 mode ad-hoc essid my-net channel 01
exit 0

and add 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.