Using multiband monopole antenna?
Unfortunately it's a little more complicated than that.  Notice on the datasheet that each frequency has a little schematic showing the required matching circuit: 

This antenna is not a broadband matched antenna - it will only work well at the single frequency that you use the matching circuit for.  So you can get it to work EITHER at 868MHz or at 2.4GHz, but not both simultaneously.  

You may want to consider a true broadband antenna instead, such as PA.710.A.  

Now, consider how you are going to connect the antenna to the two chips.  You could just connect the RF trace from the antenna to both chips, but you will get unpredictable mismatch because you don't know what the impedance is of the other chip when it's not being used.  Will it work to some extent - yes.  But I would not recommend it.  The better way to do it is to connect them using either a SPDT switch or a diplexer (maybe DPX202700DT-4035B5).  Personally I would use the diplexer, because you won't need a switch control line and because it will let you use both chips simultaneously.