Automount disk image when external drive attached and mounted

I have a 3Tb external USB 3.0 drive. Rather than partition it and end up with fixed sizes, I created a single 3Tb partition on it. I then created two sparse bundles on it... one for Time Machine and one for Google Drive. Both tend to grow in size, but not linearly, and I'm not a fan of wasted disk space. Since these grow dynamically, I can still store data on the main 3Tb volume if needed (perhaps when moving data to another system). The Time Machine volume will tend to grow and grow, but honestly, I don't have a lot of change of files... mostly office type docs, etc. we're talking Kb's to Mb's per day, not Gb's. The Google Drive volume will grow and shrink as its used (yes, I know the spare bundle will grown automatically, but I'll need to use Disk Util to shrink it down on occasion and regain free space). But in setting this up I've hit a snag... I still have to manually mount the two disk images. In the past I've used Do Something When. It would monitor things in the background and had a very basic rule: "if this drive is mounted, then mount these disk images". Pretty straight forward. However, DSW's last update was in 2007. And its still a 32bit pref pane. I understand it still works on Mavericks so long as I disable the security pref's related to sandboxing. But I'm wondering if there's perhaps a better option? Is there a newer, more equivalent to DSW? I could do this with an Automator script, but its still a manual process (just one step instead of two... right now its 1) plug in drive, 2) mount image, 3 mount image... with Automator its still 1) plug in drive, 2) run automator script). It would be awesome to have a tool or app that could function like DSW and automount my two disk images when the external drive is connected. Any thoughts? Oh, and both disk images are encrypted... the keys are saved in Keychain, so I can mount them in the GUI w/o issue, but a CLI tool isn't going to work as well if its waiting for me to enter a key...

That would be a good idea if the external drive was always with me. I guess I should mention I'm on a laptop and the drive is typically on my desk. I only have it plugged in Mon-Fri during work hours. Further, on login is a problem as I dont login in all that much. I tend to let my Mac sleep (and may disconnect a drive first), but once it comes out of sleep, items in your login list aren't processed... not unless you actually logged out or rebooted/shut down. As mentioned, I need a solution similar to what DSW did... or a local version of IFTTT that can say "okay, external drive connected, now mount the disk images on the external drive"

Thank you! I'd forgotten about Control Plane. I recall it being a replacement option for Marco Polo... and yes, I think it can do what I need. Thanks!!!