WoWInterface - Hooking a hooked function (tooltips specifically)

I've seen a few posts on hooking and such and they have all worked just fine. Unfortunately, I haven't seen any covering or how to deal with functions that are already hooked by another mod. What I'm trying to do is hook the UnitFrame_OnEnter function, so that I can let my mod have some TipBuddy support. The mod I'm working on is a unit frame mod so I need to have a custom UnitFrame_OnEnter for each frame (such as target, party1, party2, etc) unless there is a simple way of sending the UnitFrame_OnEnter function a specific this.unit value, in which case, screw the hooking. My most recent attempt looks like this.

Code:

local Original_UnitFrame_OnEnter = nil;



function Perl_Target_Tooltip_Hook()

        Original_UnitFrame_OnEnter = UnitFrame_OnEnter;

        UnitFrame_OnEnter = Perl_Target_Replacement_UnitFrame_OnEnter;

end



function Perl_Target_Tooltip_Unhook()

        UnitFrame_OnEnter = Original_UnitFrame_OnEnter;

end



function Perl_Target_Replacement_UnitFrame_OnEnter()        -- Taken from 1.8 UnitFrame_OnEnter()

        this.unit = "target";  --this is the whole reason i'm doing the hook, this one variable

        if ( SpellIsTargeting() ) then

                if ( SpellCanTargetUnit(this.unit) ) then

                        SetCursor("CAST_CURSOR");

                else

                        SetCursor("CAST_ERROR_CURSOR");

                end

        end



        GameTooltip_SetDefaultAnchor(GameTooltip, this);

        -- If showing newbie tips then only show the explanation

        if ( SHOW_NEWBIE_TIPS == "1" and this:GetName() ~= "PartyMemberFrame1" and this:GetName() ~= "PartyMemberFrame2" and this:GetName() ~= "PartyMemberFrame3" and this:GetName() ~= "PartyMemberFrame4") then

                if ( this:GetName() == "PlayerFrame" ) then

                        GameTooltip_AddNewbieTip(PARTY_OPTIONS_LABEL, 1.0, 1.0, 1.0, NEWBIE_TOOLTIP_PARTYOPTIONS);

                        return;

                elseif ( UnitPlayerControlled("target") and not UnitIsUnit("target", "player") and not UnitIsUnit("target", "pet") ) then

                        GameTooltip_AddNewbieTip(PLAYER_OPTIONS_LABEL, 1.0, 1.0, 1.0, NEWBIE_TOOLTIP_PLAYEROPTIONS);

                        return;

                end

        end

        

        if ( GameTooltip:SetUnit(this.unit) ) then

                this.updateTooltip = TOOLTIP_UPDATE_TIME;

        else

                this.updateTooltip = nil;

        end



        this.r, this.g, this.b = GameTooltip_UnitColor(this.unit);

        --GameTooltip:SetBackdropColor(this.r, this.g, this.b);

        GameTooltipTextLeft1:SetTextColor(this.r, this.g, this.b);

end

Then I just call UnitFrame_OnEnter(); in the OnEnter section in the XML and the hook works fine, but when I do this I end up breaking TipBuddy's hook it appears. Any ideas on how to do this correctly would be greatly appreciated.