@beelee
This really depends on the type of 2-unit support.
Combined Arms support: Infantry, Armor and Fighter. Only when all 3 types of units are together will they receive a bonus.
<!-- Infantry Combined Arms -->
<!-- Support given to all Armor and Fighters -->
<attachment name="supportAttachmentInfantryCombinedArmsBuff" attachTo="Infantry" javaClass="UnitSupportAttachment" type="unitType">
<option name="unitType" value="Armor:Fighter"/>
<option name="faction" value="allied"/>
<option name="side" value="offence"/>
<option name="dice" value="strength"/>
<option name="bonus" value="1"/>
<!-- Very high number is used because all unit can be buffed -->
<option name="number" value="999"/>
<!-- All Buffs need to have different names so that they can stack -->
<option name="bonusType" value="InfantryCombinedArmsSupportBuff" count="1"/>
<option name="impArtTech" value="false"/>
<option name="players" value="Germans"/>
</attachment>
<!-- Infantry Combined Arms Debuff -->
<attachment name="supportAttachmentInfantryCombinedArmsDebuff" attachTo="Infantry" javaClass="UnitSupportAttachment" type="unitType">
<option name="unitType" value="Armor:Fighter"/>
<option name="faction" value="allied"/>
<option name="side" value="offence"/>
<option name="dice" value="strength"/>
<option name="bonus" value="-1"/>
<!-- Very high number is used because all unit can be buffed -->
<option name="number" value="999"/>
<!-- All Debuffs need to have the same name so that they cannot stack -->
<option name="bonusType" value="CombinedArmsSupportBuff" count="1"/>
<option name="impArtTech" value="false"/>
<option name="players" value="Germans"/>
</attachment>
Armor units have the same type, but only for Infantry and Fighters, and Fighters buff Infantry and Armor.
How this works: If only one unit type is in a territory, those unit do not self-buff, so there is no change to their attack. When two different unit types occupy the same territory, they will buff each other, but they also debuff each other, so there is no change to the attack value. But when all three types of units are together, each type will receive 2 buffs, but can only stack 1 debuff. So, each units attack value is increased by one.
Helpful?
Cheers...