In a microwave safe bowl melt the milk chocolate chips on medium heat for about 1 minute, check the chocolate by stirring it, if not melted you can turn on again for 30 seconds. Check and stir again. Repeat if necessary. The chocolate should be smooth.
In a second microwave safe bowl combine the shortening and peanut butter chips and microwave on medium heat for 1 minute, check the peanut butter chips by stirring it, if not melted you can turn on again for 30 seconds. Check and stir again. Repeat if necessary. The peanut butter chips should be smooth.
Spoon each one alternately in lines on the pan you line with parchment paper. Just add it randomly, doesn't need to be any rhyme or reason to it. Continue until all the melted chocolate and peanut butter is on the sheet. Now take a butter knife and start to pull it through the colours to create a marble effect. But design yours any way your little heart desires.