Your detect condition is wrong, you see the way the detect condition works is if it returns false then it will be installed. In this specific case:
<ExePackage Id="AccessDatabaseEngine4Fullx64"
DisplayName="Microsoft Access Database Engine(x64)"
DownloadUrl="http://download.microsoft.com/download/2/4/3/24375141-E08D-4803-AB0E-10F2E3A07AAA/AccessDatabaseEngine_x64.exe"
Compressed="no"
Cache="yes"
PerMachine="yes"
Permanent="yes"
Protocol="netfx4"
Vital="yes"
SourceFile=".\Access\AccessDatabaseEngine_x64.exe"
DetectCondition="NOT AcccessDatabaseEnginex64 OR NOT AcccessDatabaseEnginex"/>
Burn will only install this package if the detect condition is false meaning that it will only try to install it if both packages are already installed. The correct way would be:
<ExePackage Id="AccessDatabaseEngine4Fullx64"
DisplayName="Microsoft Access Database Engine(x64)"
DownloadUrl="http://download.microsoft.com/download/2/4/3/24375141-E08D-4803-AB0E-10F2E3A07AAA/AccessDatabaseEngine_x64.exe"
Compressed="no"
Cache="yes"
PerMachine="yes"
Permanent="yes"
Protocol="netfx4"
Vital="yes"
SourceFile=".\Access\AccessDatabaseEngine_x64.exe"
DetectCondition="AcccessDatabaseEnginex64 OR AcccessDatabaseEnginex"/>
Now it will install Access Database Engine X64 only if any of the two version AREN'T installed already.