I've been looking over HOD for almost 2 years now...along with everything else out there. My little girls are sixteen months apart, with one turning 4 in early August this summer, and her sister turning 2 this past December. After oodles of research, I know that I want to follow a Charlotte Mason approach. I really struggle with whether or not to combine them when they are so close in age, or whether I should not do formal schooling until age 6, or whether I should do Ambleside Online or HOD. It. all. looks. wonderful.
One option would be: start G in LHTH this fall when she is newly 4, and let S join in when she wants. They would be 2 grades apart in public school due to birthdays, but if we didn't register G until age 6 that would make a one year grade difference. Option 2: wait until G is 5 and S is 4 to do LHTH, adding phonics, math, and handwriting for G. When people choose this option, do they go ahead and purchase LHFHG and work out of both guides? This is probably simple but always baffles me. Option 3: Start G in LHTH at age 4, let her proceed through her own guides, and then when S is 4 (16 months later), begin her alone in LHTH. This option worries me, as it seems she will have just heard all of the material and it might be redundant. I know she will want to join in on activities with G as they do everything together.
I know I am the only one who can make a final decision, and that everything might change once I actually begin teaching and learning with them. I don't know abilities, strengths, weaknesses...much more than personalities at this point! I can tell G is a perfectionist who gets frustrated easily and S is relaxed and extremely verbal for her age.
These boards are always such an encouragement to those of us beginning this journey. I really appreciate the advice and hearing others' stories! My husband was so amused by me last year when I saw the HOD booth at the Greenville, SC convention. I was nervous to go over and talk to Carrie because to homeschooling mamas, these fabulous authors are our celebrities! She was wonderful, of course.
Blessings,
Jessica
Thanks for sharing about your dc. It really does sound like the two of them can be combined, and like you would enjoy that. I agree with farmfamily, I'd start "Little Hands to Heaven" with them, but move through it a little more slowly. You could actually do LHTH, LHFHG, Beyond, and Bigger Hearts just 4 days a week and finish them exactly in 5 years if you wanted. No matter how you slow things down, which fits BTW very nicely with your line of thinking about starting slowly, I'd not get to Little Hearts for His Glory with your younger dd until she is at least 4 1/2 yo. That would be the absolute earliest I'd start a child on LHFHG. Likewise, I'd not start Bigger Hearts for His Glory until your younger dd is at least 7 yo or older. 
Starting early offers some wonderful pacing flexibility. I highly recommend it! 
It never even occurred to me to combine them and do the first 4 guides at 4 days a week. Then we could use that fifth day for art, nature study, more outside time, free reading...I am so excited!